This present application relates generally to combustion systems within combustion or gas turbine engines. More specifically, but not by way of limitation, the present application describes novel systems, apparatus, and/or methods related to downstream or axially staged injectors and/or combustion systems that use such injectors.
As will be appreciated, the efficiency of combustion or gas turbine engines (“gas turbines”) has improved significantly over the past several decades as advanced technologies have enabled increases in engine size and higher operating temperatures. The technical advances that have allowed such achievements include new heat transfer technologies for cooling hot gas path components as well as new more durable materials. During this time frame, however, regulatory standards have been enacted that limit the emission levels of certain pollutants. Specifically, the emission levels of NOx, CO and UHC—all of which are sensitive to the operating temperature and combustion characteristics of the engine—have become more strictly regulated. Of these, the emission level of NOx is especially sensitive to increases at higher engine firing temperatures and, thus, this pollutant has become a significant limit as to how much further firing temperatures might be increased. Because higher operating temperatures generally yield more efficient engines, this hindered further advances in efficiency. Thus, performance limitations associated with conventional combustion systems became factor limiting the development of more efficient gas turbines.
One way in which the combustion system exit temperatures have been increased, while still also maintaining acceptable emission levels and cooling requirements, is through the axially staging fuel and air injection. This typically requires increasing air volume passing through the combustor as well as directing more of that volume to staged injectors axially spaced downstream relative to the forward injector positioned at the forward end of the combustor. As will be understood, this increased volume of airflow results in more significance being placed on the aerodynamic performance of the unit. As a result, primary and staged injectors that minimize pressure drop of the compressed air moving through the combustion system may achieve performance benefits and efficiencies that, as flow levels through the combustors increase, become of greater significance.
Another issue related to staged injectors relates to how poorly conventional designs perform in regard to premixing fuel and air before delivery to the combustion zone. More specifically, given the tight spatial constraints and competing design criteria, conventional staged injectors typically do not properly condition the flow of air entering the device, and this leads to uneven flow characteristics that negatively impact the fuel/air mixture that exits the device on the backend. As will be appreciated, such uneven fuel/air mixtures may result in uneven combustion characteristics that generally increase the rate of engine degradation, decrease engine efficiency, and raise undesirable emission levels. As a result, a primary goal of advanced combustion system design remains the development of staged injector configurations that enable higher firing temperatures and more efficient performance, while still minimizing combustion driven emissions and aerodynamic pressure losses. As will be appreciated, such technological advances would result in improved engine efficiency levels.